This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Human metapneumovirus (hMPV), a newly-recognized member of the Paramyxoviridae family, mediate serious lower respiratory tract illness in very young children, the elderly and immunocompromised patients (1-3). hMPV causes bronchiolitis, pneumonia and flu-like syndromes, as well as asthma exacerbations, and it is considered a substantial public health problem. Since its discovery in 2001, many studies have provided a better understanding of the epidemiology associated with this viral infection. However, fundamental questions regarding the pathogenesis of hMPV-induced lung disease and the host immune response are not fully understood. Furthermore, no treatments or vaccines for hMPV are currently available. The lung innate immune system represents a critical component of the host defense against viruses and is coordinated at the cellular level by activation of transcription factors that regulate the expression of inducible gene products with antiviral and/or inflammatory activity. Key to the development of novel therapies for hMPV infection is in- depth understanding of hMPV disease pathogenesis and the nature of virus-host cell interactions that lead to either infection or killing of the virus. In this respect, the study in the interaction between hMPV and innate immune cells are of particular interest. Type I Interferon (IFN) is known as key component of the innate immune system responsible not only for broad cellular antimicrobial activity in response to viral infections, but also for its role in linking innate and adaptive immune response (4). Using an experimental mouse model of hMPV infection (5,6) and primary human cells in vitro (7), I have demonstrated that type I IFN plays a fundamental role in regulating hMPV pathogenesis. More importantly, hMPV interferes with the Toll-like receptor (TLR)- induced type I IFN production in vitro (7,8) and in vivo (5). However, the mechanisms of activation and regulation of type I IFN by hMPV infection have not yet been elucidated. Our overarching hypothesis is that hMPV activates distinct innate signaling pathways in antigen presenting cells, thus triggers immune responses that lead to efficient viral clearance and elimination of the disease. This hypothesis is based on the discovery that hMPV activates type I IFN production in human plasmacytoid dendritic cells (pDC) through the endosomal compartment where TLR7/8/9 can be activated. In addition, the cytosolic protein MDA5 is necessary for the production of type I IFN in human myeloid DC (preliminary data). In this grant, we propose to identify the mechanism(s) by which hMPV activates TLR-dependent and - independent cellular signaling in vitro and in vivo. We will achieve these objectives through the following specific aims: Aim 1. To define the molecular mechanism(s) by which hMPV induces type I IFN production in primary human cells. We will investigate how hMPV activates TLR signaling pathway to induce type I IFN in pDC and which pathway is activated by hMPV in moDC and monocytes. Aim 2. To determine the role of lung macrophages and DC in the production of type I IFN in hMPV infection. We will investigate the relative contribution of macropages and DC in the production of type I IFN in vivo using an experimental mouse model of infection. Furthermore, we will determine the contribution of pulmonary macrophages and DC in hMPV pathogenesis. Aim 3. To investigate the biological relevance of TLR-dependent and independent pathways in hMPV infection in vivo. We will characterize the virulence and pathogenicity of hMPV in mouse model of infection using transgenic mice deficient in TLR7, MyD88, and MDA5 expression. We will assess viral replication, clinical disease, lung function and inflammation, and cytokine/chemokine/IFN expression.